The present invention relates generally to the art of welding and more specifically to ac MIG welding.
There are many known processes for welding, and each process has strengths and weaknesses. Users generally select the type of process based on characteristics of the welding job, such as speed, quality, environment, material to be welded, etc.
One well known process is submerged arc (sub arc) welding, which involves the arc being submerged beneath flux. The arc is established between a consumable electrode and the workpiece and moves along a weld path. Flux is deposited along the work path ahead of the arc and the flux (and the workpiece along the weld path) is melted by the heat of the arc. Sub arc welding is known for high deposition (a large amount of flux is melted) but not all of the flux is used, and cleaning the slag and excess flux after the weld is completed takes time and can be troublesome. Also cleaning the slag requires a wider groove, typically an angle of about 50 degrees. Sub arc welding has been used to deposit 30 pounds per hour on high current welding.
Another known process is MIG welding. MIG welding is typically performed with dc and involves a consumable electrode (wire) being fed to the arc. The arc melts the wire along the weld path. MIG welding is not as fast as sub arc welding, but does not have the excess flux to be cleaned that sub arc has. DC MIG welding has been known to deposit 17-20 pounds per hour. The arc plasma column in dc MIG may cause the arc to dig or tunnel excessively and ultimately produce lower quality welds if an attempt is made to use greater deposition rates.
DC MIG welding systems can be relatively low cost, because they can have simple power circuits. MIG welding (or GMAW), as used herein, includes an arc welding process which joins metals by heating them with an arc. The arc is between a continuously fed filler metal (consumable) electrode and the workpiece. Externally supplied gas or gas mixtures provide shielding. MIG welding often is performed by welding along a weld path that is a groove along the workpieces to be joined.
MIG has typically been performed using dc current. AC current is rarely used in part, because the ac MIG process can be difficult to maintain through a zero crossingxe2x80x94the current must pass through zero at the end of each half cycle, and this can cause the MIG arc to extinguish.
Some prior art systems have used an inverter for ac MIG welding, which can have very rapid zero crossings, thus helping to stabilize the arc. However, inverters can be expensive, particularly at higher currents. Such prior art systems have not been able to deposit as much material as dc MIG systems.
The composition of the wire affects the ability of the system to maintain the arc during zero crossing. The wire may be flux cored, or metal cored. Many prior art wires exacerbate the zero crossing problem in ac MIG welding.
Accordingly, a welding system that is as fast as, or faster than, a sub arc system, but does not have the drawbacks of a sub arc system, is desirable. Preferably the groove on the workpieces to be welded may be narrow, to facilitate faster and better welds. Also, a wire that is suitable for ac MIG welding is desirable.
According to a first aspect of the invention a method of MIG welding includes providing ac power to a weld. The ac power has negative and positive portions. The negative portion is greater than the positive portion, and the frequency of output is at least 30 or 60 Hz.
According to a second aspect of the invention a method of MIG welding includes providing ac power to a weld following a weld path that includes a groove having an angle of less than 50 degrees.
According to a third aspect of the invention a method of MIG welding includes providing ac power to a weld and providing a consumable, cored, wire to the weld.
According to a fourth aspect of the invention a method of MIG welding includes providing ac power to a weld, and providing a consumable wire to the weld at a rate of at least 30 or 35 pounds per hour using a single arc.
According to a fifth aspect of the invention a method of MIG welding includes providing ac power to a weld wherein the negative portion is at least 1.5 times the positive portion.
According to a sixth aspect of the invention a method of MIG welding includes providing ac power to a weld. The negative portion is greater than the positive portion and weld process begins with a negative portion of at least 0.5 seconds duration.
According to a seventh aspect of the invention an apparatus for MIG welding a substrate of low carbon steel having a thickness less than xc2xcxe2x80x3 comprises a table for holding the substrate and a MIG gun moved over the substrate by a carriage, depositing wire at a rate of at least 30 pounds per hour while the MIG gun is welding. A source of shielding gas provides shielding gas to the MIG gun and a power source provides a substantially square AC current at the MIG gun, with the average current being at least 300 amps.
According to an eighth aspect of the invention a system of MIG welding includes an ac power source that has a MIG output and a controller that controls the power source. A feedback circuit is between the power source and the controller. A source of consumable wire provides wire to the MIG output. The controller provides that the negative portion is greater than the positive portion, and that the output frequency is at least 60 Hz.
According to a ninth aspect of the invention a system of MIG welding includes a MIG torch, an ac power source that provides ac power to the MIG torch and a source of consumable wire that provides wire to the MIG torch. The wire is made of a sheath encapsulating a core comprised of up to 5% by weight, or 0.3% to 5% by weight, graphite and one or more compounds of potassium.
According to a tenth aspect of the invention a system of MIG welding includes an ac power source having a control input and a MIG output. A controller has a balance circuit and a feedback circuit, and controls the power source such that the negative portion is at least 1.5 times the positive portion.
The unbalance can be the result of duration, current, amp-seconds, or other parameters.
The frequency is between 90 Hz and 120 Hz in other embodiments.
A consumable, flux-cored, metal cored, or non-cored wire is provided to the weld in various alternatives. The metal cored wire can be a sheath encapsulating a core, and the core is a combination of graphite and one or more compounds of potassium, and the combination does not exceed approximately 5% by weight, or is between 0.3% and 5% in other embodiments. The compound of potassium is K2MnTiO4 in another embodiment.
The weld is performed on a workpiece having a groove having an angle of less than 50 degrees, 20-30 degrees, or less than 30 degrees in a number of embodiments.
The process is performed to deposit wire at a rate of at least 35 pounds per hour using a single arc, or at least 40 pounds per hour in other embodiments.
The negative portion is 1.5 times, or twice, the positive portion in various embodiments.
The process is initiated with a negative having a duration of at least 0.5 seconds, or at least 0.75 seconds in other alternatives.
The process is preformed with a stick-out of about 2 inches, and using a shielding gas at a rate of at least 80 cubic feet per hour. In other embodiments.
According to another aspect of the invention the dilution of MIG welding process is controlled by using ac power and controlling the balance to obtain a desired dilution. The balance may be greater EP or greater EN.